matroskadec: Fix a bug where a pointer was cached to an array that might later move due to a realloc()

Fixes bug #190
Chromium bug #100492
related to CVE-2011-3893

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

(cherry-picked from commit faaec4676c)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>
(cherry picked from commit 1f625431e2)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavformat/matroskadec.c

@@ -1074,13 +1074,13 @@ static void matroska_convert_tags(AVFormatContext *s)
 static void matroska_execute_seekhead(MatroskaDemuxContext *matroska)
 {
     EbmlList *seekhead_list = &matroska->seekhead;
-    MatroskaSeekhead *seekhead = seekhead_list->elem;
     uint32_t level_up = matroska->level_up;
     int64_t before_pos = url_ftell(matroska->ctx->pb);
     MatroskaLevel level;
     int i;
 
     for (i=0; i<seekhead_list->nb_elem; i++) {
+        MatroskaSeekhead *seekhead = seekhead_list->elem;
         int64_t offset = seekhead[i].pos + matroska->segment_start;
 
         if (seekhead[i].pos <= before_pos